Polyolefins such as polypropylene (PP) have been used in many applications in the form of molded articles, film, sheets, etc., because these plastics are excellent in molding processability, toughness, moisture resistance, gasoline resistance, and chemical resistance, have a low specific gravity, and are inexpensive. However, polyolefins are poor or inadequate in heat resistance, stiffness, impact resistance, and scratch resistance. These deficiencies are obstacles to opening up new applications for polyolefins.
On the other hand, thermoplastic polyesters such as polyethylene terephthalate (PET) are widely used as engineering thermoplastics in the fields of automobile parts, electrical and electronic parts, because such polyesters have high heat resistance, stiffness, strength, scratch resistance, oil resistance, solvent resistance, and the like. It would, however, be desirable to further improve the molding processability, toughness, notched impact resistance, moisture resistance, and chemical resistance of these plastics. In addition, thermoplastic polyesters are disadvantageous in that these plastics have a higher specific gravity and are more expensive than polyolefins.
From such a view point, it would seem a useful approach to blend polyolefins and thermoplastic polyesters in order to obtain a thermoplastic resin having the characteristics of both of these resins for a particular application requiring aspects of each of these polymers. However, physical blending of these polymers have not been successful. Since polypropylenes and thermoplastic polyesters are naturally incompatible, mere mixing of these polymers involves problems. These immiscible plastics exhibit poor adhesion along blend interfaces, with resultant weakness. The mechanical properties, in particular, impact resistance, tensile elongation, and tensile strength of a molded product made of a mixture of polypropylene and thermoplastic polyester often have values lower than those expected by simple additivity or averaging of the physical properties of the polypropylene and thermoplastic polyester. The processability of a mixture of these polymers is limited to injection molding. The resulting products show extreme nonuniformity and ugly appearance owing to formation of flow marks, and cannot be used in practice in the manufacture of automobile parts or electric and electronic parts.
Conceptually, it should be possible to combine two dissimilar thermoplastics to obtain a blend that expresses the best properties of both polymers. A general example would be the blend of an amorphous polymer with a semi-crystalline polymer. In such a blend, the objective is to obtain the high heat and fatigue resistance of the amorphous polymer and the processability and solvent resistance of the semi-crystalline polymer.
In general, both interfacial agents and impact modifiers are required to produce multipolymer blends with a desirable balance of properties. Interfacial agents provide adhesion between the principal polymer phases, improving stress transfer, and are necessary to reduce interfacial tension during processing that can lead to gross phase separation. Thus, interfacial agents play an important role in determining the ultimate morphology of the blend.
Another objective is compatibilization and toughening of dissimilar plastic scraps produced in industrial or in recycling operations. Recycling operations, which have proliferated in recent years, are providing an abundant and inexpensive source for some thermoplastics. For instance, the recycling rate in the U.S. for plastic soft drink bottles made out of polyethylene terephthalate topped 31% in 1990. Recycled PET is particularly useful as an injection moldable material which can be formed into articles exhibiting a good balance of properties, including strength and stiffness. However, an improvement in impact strength and processability of these recycled PET materials is desirable.
Methods for improving impact strength of individual polymers include the use of hydrogenated block copolymers of styrene and butadiene, i.e., hydrogenated SBR rubbers, to enhance interfacial adhesion. These copolymers have a low glass transition and low modulus rubber phase which is required for impact modification. Hydrogenated SBR's also have little unsaturation, and can thus be blended with high processing temperature plastics without degradation.
Hydrogenated styrene-butadiene copolymers are unique compared to other rubbers in that these copolymers contain blocks which are microphase-separated over application and processing conditions. This microphase separation results in physical cross-linking, causing elasticity in the solid and molten states. Such an internal strength mechanism is often required to achieve toughness in plastic impact modification. The melt elasticity of the block copolymer during processing can, under the right conditions, enable it to be finely dispersed with another polymer in a stable, interpenetrating, co-continuous phase structure. In general, to significantly increase the impact strength of a thermoplastic, it is necessary to blend in an elastomer that forms finely-dispersed rubber particles within the plastic matrix. These rubber particles improve energy dissipation in the thermoplastic while simultaneously limiting the growth of cracks. To achieve the required morphology for effective toughening, the styrenic block copolymer should be compatible with the thermoplastic to be toughened.
While hydrogenated block copolymers of styrene and butadiene can improve the impact strength of polyolefins and polystyrene, such copolymers are less useful for modifying polymers dissimilar in structure to styrene or hydrogenated butadiene, such as polyesters and polyamides. However, impact modification using these copolymers can be further improved by grafting functional groups to the block copolymer which interact with the dissimilar material and enhance polymer blending. These interactions can include chemical bonding, e.g., cross-linking, hydrogen bonding and dipole-dipole interaction. A certain amount of residual unsaturation must be present in order to obtain an advantageous degree of functional moieties on the base copolymer.
Use of these techniques has been described in Chen U.S. Pat. No. 4,547,547, issued Oct. 15, 1985 to increase impact resistance and crystallization velocity of polyester blends by addition of a minor amount of a segmented polyesteramide. Lutz U.S. Pat. No. 4,795,782, issued Jan. 3, 1989 reports use of a functionalized elastomer and functionalized polyolefin to improve impact strength in a blend with a polyamide. Gergen U.S. Pat. No. 4,797,447, issued Jan. 10, 1989, describes use of a functionalized elastomer to improve impact strength of polyesters. Liang U.S. Pat. No. 4,952,629 issued Aug. 28, 1990, details use of functionalized elastomer to affect impact strength in a blend of polyester and polyesteramide.
Maleic anhydride has been proposed as a compatibilizing group for a variety of plastic blends; see Plastics Technology, February, 1989 pages 67-75 and Albee et al., Plastics Compounding, September/October 1990, pages 32-41. Kraton 1901X is a commercially available, maleic anhydride functionalized copolymer as described in Shell Chemical Co., Technical Bulletin SC:592-89. Compositions made from epoxy- and anhydride-modified polymers have also been proposed; see Okada U.S. Pat. No. 4,981,896 issued Jan. 1, 1991.
The performance of functionalized elastomers such as Kraton FG1901X in compatibilizing polymer blends has been mixed. Gelles et al., Proceedings of the SPE 46th Annual Technical Conference and Exhibits, pp. 513-515 (1988), used Kraton FG1901X in an attempt to compatibilize Nylon 6,6 with PPE, PP, and modified polypropylene. Non-functionalized Kraton G rubber was used as a comparison; see Table 3 at p. 514. Neither Kraton 1901X or G used at a 20% level provided any improvement in impact strength. Blends of nylon 6,6 with polypropylene and both rubber additives (40:40:10:10) produced some improvement. Overall, the authors concluded the nylon 6,6 was toughened by the Kraton 1901X rubber while the polyolefin and PPE phases are primarily toughened by the Kraton G (unmodified) rubber (at p. 514). Their experiments further showed that Kraton 1901X was effective to improve the impact strength of PET or PBT alone, but no mention was made of using Kraton 1901X in a polyolefin-PET or PBT blend, or what results might be expected with such a blend.
While impact strength in blends has undoubtedly been improved using functionalized elastomers, properties of the individual polymers in the blend have suffered. The present invention addresses this problem.